Abrasive cleaning and processing of material is frequently used as a preparatory step to prepare the surface of a material (typically metal) for other processing such as sealing and/or painting, or simply cleaning and renewing a surface. Abrasive cleaning or processing is commonly referred to as blast cleaning or blast processing, and the terms are used interchangeably herein. In general, abrasive cleaning or blast cleaning involves applying accelerated particulate materials against a surface to be cleaned. Blast cleaning can be effective in removing any existing paint, dirt, grease, oil, rust, or other surface contaminant from a target to provide a cleaned surface.
Traditionally, blast cleaning has been performed through the use of sand, grit, or other particulates as the blasting medium. The two traditional approaches to accelerating the blasting medium include either propelling it at high velocity with pneumatic means, or projecting it via mechanical means.
Use of pneumatic means is often referred to as “air blasting”, even though liquid can be used in addition to or in place of air as the propellant. Air blasting can be accomplished via suction or pressure. Suction systems use a vacuum to draw media into the air stream. Pressure systems feed media directly into a pressurized air stream that accelerates it toward the surface to be blasted. Wet blasting propels blast media in liquid suspension onto the surface to be cleaned.
The most common mechanical means is wheel blasting, where abrasive particulate matter is projected against the target surface through the use of a rapidly rotating centrifugal wheel. Wheel blasting machines typically have one or more blast wheels each having a multiple blades extending radially from the wheel. The size of the wheel blast machine and the number and power of the blast wheels vary considerably depending on the targets to be blasted as well as on the expected result and efficiency. The blast wheel is mounted in a housing and is rotated at high speed. Abrasive such as steel shot is fed onto the blades and is projected against the target surface. The impact of the abrasive with the target dislodges paint, rust, and other debris from the surface.
The equipment and methods of traditional systems are subject to a variety of deficiencies. The primary concern is that the systems cannot provide the desired finish in a single pass, or a single processing of the target. The metal condition and finish specifications are linked. The condition of the metal (i.e., the amount of scale, rust, or corrosion and the desired surface profile specification) determines the amount of blast material and the length of blasting time required to achieve the desired outcome.
The surface profile specification is based on the depth of the target's surface cross-section versus the thickness of the applied finish. The Steel Structures Painting Council (SSPC) for surface cleaning quality has set standards. As an example: “white metal blast cleaning” means that the target should be free of all visible rust, mill scale, paint, and foreign matter.
However, traditional systems either use mixtures of shot and grit that create a dimpled or peened surface profile, and/or require additional processing to achieve the desired surface profile.
A dimpled or peened surface profile does not have the necessary surface qualities for coatings to adhere. Such a profile is in contrast to the higher quality “anchor profile” that allows good coating adherence. An additional problem may exist in creating a surface that traps contaminants by shattering the mill scale rather than removing it. Such a peened surface results in the edges around the dimple rotting over and trapping mill scale. Trapping milt scale during the abrasive cleaning is referred to as “non-visual contamination”. Painting or coating over non-visual contamination results in accelerated failure of protective coatings due to the accelerated formation of corrosion.
Additional processing is typically used to compensate for the inability of traditional machines to provide the desired surface. The additional processing can either be additional, labor intensive, manual open blasting processing (for example, pneumatic (such as sandblasting) using a manually operated hand held nozzle) to achieve the desired angular profile after the mill-scale is removed.
A related problem is that traditional machines typically only prepare one side of a steel plate, pipe, or other target at a time, and then must be turned over for the process to be repeated. The alternative of running the target through a centrifugal/wheel blasting machine two or more times (at least once for each side) has a number of disadvantages. Typically, wheel blasting machines must use a combination of grit and shot, and the combination leaves a significant variation in the plate surface. Surface variation develops because there is a predisposition for the grit to clump together and separate from the shot. Also, even when used with shot, the use of grit produces accelerated wear on the internal components of the machine. The increased wear requires increased and more frequent maintenance, which increasing operating costs and reduces efficiency.
In addition to the problems described above, traditional wheel blasting and pneumatic blasting machines are generally configured for a fixed size target. There are typically no easy adaptations for use in the treating of a range of sizes of pipe or other target shape. Additionally, traditional equipment produces a tendency for abrasive particles to drop out of circulation and collect in various portions of machines and external environments, which reduces recovery of the abrasive medium and reduces overall performance efficacy.
Most conventional centrifugal wheel blast operations are “open” systems requiring a collection pit for used media. Gravity is often used to deposit used media into the collection pit after it has impacted the surface. Attempts have been made to reuse the material more effectively, which generally includes some combination of augers, brushes, scoops, elevators, and/or other collection and transport means to move the media from the collection pit to the feed hopper to be re-circulated.
One previous system, as described in U.S. Patent Application Publication No. US 2003/0064668 A1, describes a rotor system that attempts to create a vortex to continually recycle particulate material. However, that system as described is subject to multiple failures, and does not provide a functional machine for industrial use. One particular problem is that of breaking down of the machine, and not keeping the blast medium particulates in circulation as desired.
Descriptions of certain details and implementations follow, including a description of the figures, which may depict some or all of the embodiments described below, as well as discussing other potential embodiments or implementations of the inventive concepts presented herein. An overview of embodiments of the invention is provided below, followed by a more detailed description with reference to the drawings.